In recent years torsional hinged high frequency mirrors (and especially resonant high frequency mirrors) have made significant inroads as a replacement for spinning polygon mirrors as the drive engine for laser printers. These torsional hinged high speed resonant mirrors are less expensive and require less energy or drive power than the earlier polygon mirrors.
As a result of the observed advantages of using the torsional hinged mirrors in high speed printers, interest has developed concerning the possibility of also using a similar mirror system for video displays that are generated by scan lines on a display surface.
Existing CRT (cathode ray tube) video systems for displaying such scan-line signals on a display screen use a low frequency positioning circuit to synchronize the display frame rate with an incoming video signal, and a high frequency drive circuit to generate the individual image lines (scan lines) of the video. In the CRT systems, the high frequency circuit operates at a frequency that is an even multiple of the frequency of the low speed circuit and this relationship simplifies the task of synchronization.
Therefore, it would appear that a very simple corresponding torsional hinged mirror display system would use a first torsional hinged high speed scanning mirror to generate scan lines and a second slower torsional hinged mirror to provide the orthogonal motion necessary to position or space the scan lines to produce a raster “scan” similar to the raster scan of the electron beam of a CRT. Unfortunately, the problem is more complex than that. First of all, scanning motion of a high speed resonant mirror cannot simply be selected to have a frequency that is an even multiple of the positioning motion of the low frequency mirror.
Second, although a raster scan CRT system is easily controlled and sufficiently bright for most applications, the display of a corresponding raster scan mirror based system may be dim, and would benefit from an increase in brightness. For example, the modulated light beam is typically on for no more than 10 to 20% of the time. More specifically, the modulated light source of existing mirror visual systems is turned on and produces a scan line only when the mirror is moving or sweeping in one direction, (i.e. 50% of the time). Likewise, an image frame is generated only when the low speed cyclic positioning mirror is moving in one direction. Consequently, the time is reduced another 50%, thereby leaving a maximum possible “on-time” of the modulated beam of only 25%. Finally, since the oscillating mirrors travel in one direction, stop and turn around and then travels in the opposite direction, these turn-around portions (or peak points of the sinusoidal movement) are unsuitable for displaying images. As an example only, if the oscillating mirror has an overall or average frequency of 60 Hz or 20 kHz, yet must slow down, come to a complete stop, and then accelerate in the opposite direction each time the beam sweeps across a display, it will be appreciated that the angular velocity of the mirror movement is anything but constant. However, to generate an undistorted image from periodically received pixels, the velocity of an oscillating mirror during the display portion of its travel should be substantially constant. Consequently, as much as 50% of the mirror movement that is located at turn around or peak portions cannot be used, which leaves potentially less than about 10% of the total time that the modulated light beam is generating an image.
Based on the foregoing discussion, an immediate and easy solution to the brightness problem would appear to only require the system to generate another image frame during the unused half of the cyclic motion of the slow speed positioning mirror, or alternately, that a scan line be generated for each back and forth sweep of the resonant mirror rather than during a sweep in only one direction. This would double the brightness. Alternately, the unused half of the mirror travel of both mirrors could be used to increase the brightness of the image by a factor of four.
According to the present invention, the image brightness is doubled and the quality of the image improved by using both directions of the bi-directional beam sweep of the high speed resonant mirror to generate a scan or image line, and is applicable for use with both visual display systems and laser printer systems. Unfortunately, the problem is not solved by simply deciding to generate a scan line in both directions of the bi-directional beam sweep. The difficulty is aligning the two consecutive scan lines for an acceptable display.
However, in addition to aligning the two consecutive scan lines formed by the bi-directional sweep when used with visual display systems, the positioning motion of the low frequency mirror and, consequently, the low frequency drive signal must also be synchronized with the image frame rate of the incoming video signals to avoid noticeable jumps or jitter in the display. At the same time, however, the high frequency mirror, whether used with a visual display or a laser printer, must run or oscillate at substantially its resonant frequency, since driving a high-Q mirror at a frequency only slightly different than the resonant frequency will result in a significant decrease in the amplitude of the beam sweep (i.e. reduce the beam envelope). This would cause a significant and unacceptable compression of the image on the display. Therefore, for visual display systems, the high speed mirror drive is decoupled from the low speed mirror drive. That is, as mentioned above, the high speed drive signal cannot simply be selected to be an even multiple of the low speed drive signal.
Further, in a digital imaging system, each frame or image of incoming signals representing image pixels (such as might be received from a computer hard drive, a TV station, a DVD player or a VCR player) must still be faithfully reproduced. This means, each pixel of each successive image (or printed page) must be properly located on the screen of the display (or the printed page) in both directions if distortions are to be avoided. Also of course, if complete images or complete scan lines are lost or dropped, glitches or artifacts in the display would clearly be observed. Therefore, as described above in a torsional hinged mirror based video system, the low frequency mirror drive must still be synchronized to the flow rate of the incoming video signals. At the same time, however, the high speed mirror, whether used in a visual display or printer, must still oscillate at substantially its resonant frequency. The problems discussed above are even further complicated if there has been some degradation of the image signals. For example, if the source of the video signals is a VCR, one common problem such as stretching of the VCR tape could vary the incoming frame rate, which must also be dealt with. Additionally, tracking or synchronizing the low speed mirror and the frame rate should be done in a way that minimizes transients from discontinuities in the drive waveform.
Therefore, a mirror based imaging system having increased brightness and that overcomes the above mentioned problems would be advantageous, but doubling the beam “on time” by generating scan lines in each direction of the high speed bi-directional beam sweep presents many difficult challenges.